6. PROJECT SUMMARY / ABSTRACT Dysfunctional neuronal circuits are thought to be responsible for neuropsychiatric disorders, including obsessive-compulsive disorder, Parkinson[unreadable]s disease and addiction. G protein-coupled receptors (GPCRs) differentially modulate the activity of neuronal circuits by interacting with different G proteins, (e.g. G{as}, G{ai}, and G{aq}-families), and these receptors are well-established drug targets. Distinct subpopulations of neurons comprising these circuits often express similar receptor subtypes, making it impossible to create drugs that selectively target receptors in specific circuits. In order to determine how receptors might modulate neuronal circuits, it is necessary to selectively activate GPCR signaling in a specific neuronal population in vivo. For the past two decades, efforts have been made to create tools to selectively manipulate GPCR signaling in distinct neuron populations - from small molecule ligands to genetic deletion - but these approaches have off-target effects. These off-target effects effectively eliminate the possibility to determine how GPCR signaling in a specific neuronal population affects circuitry function. A promising advancement to enable this specific manipulation is the development of the Designer Receptors Exclusively Activated by Designer Drug (DREADD) family of receptors. The objective of this proposal is to validate this recently created tool in vivo using a transgenic mouse expressing the G{as} DREADD in a specific population of neurons in the striatum. Specific Aim 1 will validate the tool by determining whether the G{as} DREADD activates canonical G{as} signaling pathways by measuring cAMP accumulation, ERK1/2 phosphorylation, DARPP-32 phosphorylation, and c-Fos expression. Specific Aim 2 will validate the physiological relevance of the G{as} DREADD-induced signaling by assessing its effects on animal behavior. The completion of this proposal will provide neuroscientists a powerful new tool to exclusively modulate GPCR signaling in specific neuronal populations.